1. Field of the Invention
The present invention relates to a highly porous ceramic fabricated from expandable microspheres and a preceramic polymer, and a method for fabricating the highly porous ceramic. More particularly, the present invention relates to a highly porous ceramic having a high porosity of not less than 60% and a pore density of not less than 108 pores/cm3 fabricated using expandable microspheres and a preceramic polymer as starting materials wherein the size and the distribution of the pores are uniform and the porosity can be easily controlled, and a method for fabricating the highly porous ceramic. The highly porous ceramic thus fabricated can be suitably used for various structural materials, refractory materials, kiln furniture, insulating materials, shock-absorbing materials, bulletproof materials, lightweight structural materials, and so on.
2. Description of the Related Art
In general, porous ceramics are materials which can be used in a wide range of applications, such as various high temperature structural materials, high temperature refractory materials, kiln furniture, shock-absorbing materials, bulletproof materials, lightweight structural materials, insulating materials, and the like.
When the porous ceramics have a low porosity, they are disadvantageous in terms of poor insulating properties, low specific strength (strength per unit weight) and low cost-effectiveness of materials.
In addition, when the porous ceramics have a non-uniform pore size distribution or non-uniform distribution of pores in the porous ceramics, stresses applied to the porous ceramics cannot be equally distributed throughout the ceramics. Accordingly, the applied stresses are concentrated on more porous region, thus deteriorating the strength of the porous ceramics and making the strength distribution throughout the porous ceramics non-uniform.
Further, in the case that the porous ceramics are used as insulating materials and refractory materials, non-uniform pore size distribution or non-uniform distribution of pores in the porous ceramics results in poor heat transfer properties, which makes thermal gradient in the porous ceramics non-uniform.
Accordingly, high porosity of porous ceramics and uniformity of pore distribution and size are important in terms of the performance and quality of porous ceramics.
Generally, the porous ceramics are fabricated in accordance with two procedures as follows:
First, a ceramic is mixed with a pyrolyzable material or volatile material. Thereafter, gases are evolved by the pyrolysis of the pyrolyzable material or volatilizing the volatile material, and the evolving gases form pores in the ceramic to fabricate a porous ceramic (see, e.g., U.S. Pat. Nos. 5,358,910 and 5,750,449).
In summary, after a ceramic and a preceramic polymer are mixed with each other by a ball milling process, the mixture is molded into a desired shape. The molded body is heated to fire combustible components and volatilize volatile components contained in the preceramic polymer (pyrolysis). The ceramic components contained in the molded body are sintered by heating, and the volatile components contained in the preceramic polymer are volatilized to form pores within the molded body, thereby fabricating a final porous ceramic.
However, this method has a disadvantage that when the content of the polymeric components is not less than 50%, the shape of the molded body may collapse due to softening and pyrolysis of the polymeric components. Accordingly, it is difficult to fabricate highly porous ceramics having a porosity of 70% or more. Further, uniform distribution of pores is difficult to obtain and pore size cannot be easily controlled according to the material properties.
Second, a porous ceramic can be fabricated by lowering the sinterability of a ceramic. This method is divided into the following two procedures. The first method is carried out by sintering a ceramic below optimum sintering temperature to lower the relative density of the ceramic, thereby forming more pores within the ceramic. However, since the porous ceramic thus fabricated is not sintered at optimum sintering conditions, mechanical properties such as strength may be greatly deteriorated.
Additionally, U.S. Pat. No. 6,214,078 discloses a method for fabricating a porous ceramic. According to this method, first relatively coarse grains and relatively fine grains are mixed with each other. After the mixture is molded into a molded body having a particular shape, the molded body is sintered by heating. At this time, since the relatively coarse grains have a relatively low surface energy that acts as a driving force for the sintering, they impede the sintering. Since the relatively fine grains have a relatively high vapor pressure, they tend to evaporate and condense on the relatively coarse grains. The condensation of the relatively fine grains lowers the relative density to fabricate a porous ceramic. However, this method has disadvantages in that it is difficult to fabricate highly porous ceramics having a porosity of 50% or more, and the pore size is non-uniform.
On the other hand, U.S. Pat. Nos. 5,158,986 and 5,334,356 disclose methods for producing microcellular plastic materials, although they are not directed to ceramics.
According to these methods, CO2 in a supercritical state is introduced into a plastic material to saturate the plastic material, and then rapidly depressurized to evolve supersaturated CO2 from the plastic material. The evolving CO2 forms micropores in the plastic material to produce a microcellular plastic material.
The microcellular plastic material produced by using CO2 in a supercritical state as a blowing agent for forming micropores has uniformly distributed micropores throughout the plastic material.
However, since CO2 is substantially insoluble in ceramics, the above methods cannot be applied to ceramics. In addition, the methods involve complex process operations, such as saturation of the polymer plastic material in an autoclave, depressurization, curing, pyrolysis, etc.